Red mud is a solid waste product generated during a process for the production of alumina. For example, red mud is produced during the Bayer process for alumina production, the principal industrial means of refining bauxite in order to provide alumina as raw material for the electrolysis of aluminum by the Hall-Héroult process. A typical plant produces one to two times as much red mud as alumina. This ratio is dependent on the type of bauxite or ore used in the refining process.
Generally, red mud comprises mixture of solid and metallic oxide-bearing impurities, and presents one of the aluminum industry's most important disposal problems. The red colour can be caused, for example, by the oxidised iron present therein. Red mud cannot be disposed of easily. In most countries where red mud is produced, it is pumped into holding ponds. Red mud is thus a problem since it takes up land area and can neither be built on nor farmed, even when dry. Red mud is, for example, highly basic. For example, the pH can be ranging from 10 to 13. Several methods have been developed to lower the alkaline pH to an acceptable level to decrease the impact on the environment. Several researches have been done to find a suitable way to utilize the mud for other applications, but drying the mud requires much energy (latent heat for water evaporation) and can represent high costs if fossil fuels have to be used in the drying process.
The quantities of red mud worldwide continue to grow. In 2010 alone, 80 million tonnes of alumina were produced throughout the world, creating over 120 million tonnes of red mud. It is estimated that red mud inventory in the world has actually reached well over 2.5 billion tonnes. This figure will only continue to grow as increasing demand for aluminium drives the demand of alumina, and in turn for bauxite, which means increased production of toxic red mud residues.
There is thus a need for an alternative process for treating red mud.